Acoustic noise in large amounts can be unfavourable to human well being and ultimately to health. In some countries, government regulations exist attempting to protect employees by setting various maximum allowable dose levels and peak levels during a working day or week. Such levels relate e.g. to the levels of noise exposure averaged over a working day or week, and to the maximum noise (peak sound pressure) to which employees are exposed over a working day. In the EU, such levels are set in the “DIRECTIVE 2003/10/EC OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 6 Feb. 2003 on the minimum health and safety requirements regarding the exposure of workers to the risks arising from physical agents (noise)”.
In the EU-directive, the daily exposure limit value LEX,8h defines that the exposure in dB(A) (minus the attenuation from any hearing protectors) have to be lower than 87 dB(A) during a normal 8 hour working day. The daily exposure level LEX,8h is a time weighted average of the instantaneous noise pressure level (relative to 20 μPa) for a nominal eight hour working day (cf. e.g. ISO 1999:1990, Acoustics—Determination of occupational noise exposure and estimation of noise-induced hearing impairment). The unit ‘dBA’ or ‘dB(A)’ refers to a weighting of the signal to approximate the different sensitivity of the human auditory system at different frequencies, the ‘A-weighting’ (roughly) applying a lower weight to sounds below 1 kHz and above 4 kHz, see e.g. International standard IEC 61672:2003. Further, it is defined that the peak sound pressure ppeak at any time during the day has to be below 200 Pa, corresponding to 140 dB(C) in relation to 20 μPa. The unit ‘dBC’ or ‘dB(C)’ refers to a weighting of the signal similar to the ‘A-weighting’, but with a lower cut-off frequency at low frequencies (around 50 Hz instead of around 1 kHz).
A compressor circuit designed to improve a telephone system in a manner that improves the ability of the system to resist the transmission of a shock arising in the telephone system to the headset user is e.g. described in WO 03/079722 A1. In an embodiment, the circuit is adapted to immediately reduce the output signal when the input signal reaches a certain (high) level (e.g. above 105 dB).